This invention relates generally to a photoconductive belt assembly employed in an electrophotographic printing machine, and more particularly concerns a belt assembly comprising a sub-belt having a photoconductive belt secured releasably thereto.
In an electrophotographic printing machine, a photo-conductive belt is charged to a substantially uniform potential so as to sensitize the surface thereon. Thereafter, the charged portion of the photoconductive belt is exposed to a light image of an original document being reproduced. Exposure of the charged photoconductive belt selectively discharges the charge thereon in the irradiated areas. This records an electrostatic latent image on the photoconductive belt corresponding to the informational areas contained within the original document being reproduced. After the electrostatic latent image is recorded on the photoconductive belt, the latent image is developed by bringing a developer mix into contact therewith. The developer mix comprises toner particles adhereing triboelectrically to carrier granules. These toner particles are attracted from the carrier granules to the latent image forming a toner powder image thereon. The toner powder image is then transferred to a copy sheet. Finally, the copy sheet is heated to permanently affix the toner particles thereto in image configuration. This general approach was originally disclosed by Carlson in U.S. Pat. No. 2,297,691, and has been further amplified and described by many related patents in the art.
Generally, an electrophotographic printing machine utilizes either a photoconductive drum or belt. Various materials have been proposed for photoconductive belts or drums employed in electrophotographic printing machines. One well known material is made from a selenium alloy which is capable of producing a substantially large number of copies. Another material may be of an organic type. However, both of these materials, when used as belts, frequently pose difficulties in belt tracking. Typically, the photoconductive belt is rather thin and sensitive to edge forces. Thus, an edge guide tracking system may introduce side buckling or lateral distortion of the photoconductive belt which will significantly impair the usage thereof. Moreover, the photo-conductive belt is frequently seamed. When a seamed photo-conductive belt is employed, the position of the seam must be known so as to synchronize the placement of the electrostatic latent image and registration of the copy sheet therewith. Heretofore, this type of synchronization was achieved by having holes in the belt edge which are sensed. Alternatively, the belt holes may mesh with a sprocket wheel. The sprocket wheel would then drive a synchroneous registration member. Another technique is to mark the photoconductive belt with opaque, reflective, or magnetic indicia. This indicia would be detected and employed to control the exposure and copy sheet registration. However, all of the former approaches require additional processing of the photoconductive belt. Processing of this type increases costs and risks to the belt reliability. In addition, edge guiding poses a serious risk to the belt structural integrity.
Accordingly, it is a primary object of the present invention to improve the photoconductive belt assembly employed in an elecetrophotographic printing machine.